Biking

The Washington DC area has become a great biking destination, with more than 800 miles of jogging and biking trails in the region. Many of these are of the popular "rail to trail" sort, where previous railroad tracks have been converted to scenic bike trails.
Be aware that the best trails are frequently the most crowded, especially during peak weekend hours, so if possible, plan for early mornings and weekdays.

The Capital Crescent Trail - This 13-mile popular trail is partially paved with some crushed stone, and is another "rail to trail" project. The seven-mile portion of the trail between Georgetown and Bethesda features a nine-foot wide pavement, and the northern four-mile portion between Bethesda and Silver Spring is crushed gravel as a temporary measure, with plans to pave it in the future. To make crossing easy and painless, the train includes two tunnels that offer trail users gentle gradients and grade-separated roadway crossings. A recent survey showed that the trail is one of the most heavily used trails in the nation.

Chesapeake & Ohio Towpath - This trail is really a wonderful resource, and runs the entire 184 miles from DC to Cumberland, MD, along the towpath that follows the old man-made Chesapeake & Ohio Canal, built between 1828 and 1950. The C&O Canal has mostly been drained and the forest has taken over, but in some areas the canal still borders the towpath, which is a great trip for hikers, joggers, and bicyclists. The trail has several historic structures and sites along the way, including 19th century locks from the canal's working days and other sites such as the Great Falls, the Monocacy River Aqueduct, Harpers Ferry, and the Paw Paw Tunnel. There is now a new Great Allegheny Passage Trail that connects with the C&O in Cumberland, MD to create a continuous 318-mile long, traffic-free ride from McKeesport, near Pittsburgh, to Washington DC.

Custis Trail - This four-mile paved trail in Arlington from Bon Air Park, east to the Key Bridge at Rosslyn, has a few moderate climbs and winding turns, but nicely, almost no grade crossings. It merges with the Mount Vernon Trail along the Potomac River and the Washington & Old Dominion Railroad Trail at Bon Air Park. To reach the trail, take Metro to Rosslyn.

Mount Vernon Trail - This is an 18.5-mile paved trail along the Potomac River, running parallel to the George Washington Memorial Parkway. It extends from Mount Vernon, the home of George Washington, to Theodore Roosevelt Island in the Potomac River near the Lincoln Memorial. Along the way, enjoy a view of the Potomac at Riverside Park, visit the fortifications at Fort Hunt Park, and take a side trip to the Dyke Marsh wildlife habitat or to Jones Point Park, which features a 19th-century lighthouse. You can then lunch on the wharf in Alexandria amid the 18th-century homes and shops occupied ever since the city was a tobacco and shipping port. Beyond Alexandria, you can see the sailboats off Daingerfield Island and view the Washington skyline from Gravelly Point, where you can also watch jet planes land and take off from Ronald Reagan National Airport. Once over the Columbia Island Bridge, you pass the Navy-Marine Memorial of gulls in flight above a wave. Next, you can rest in the greenery of the Lyndon Baines Johnson Memorial Grove in Lady Bird Johnson Park where you can view the Washington monuments from a distance. The trail passes Memorial Bridge that symbolizes the union of the North and South after the Civil War. Just past Theodore Roosevelt Island is the connection to the Arlington County trail system.

National Mall Path - The 2.5 miles between the Capitol and the Lincoln Memorial features paved sidewalks and fairly flat terrain, along with beautiful landscaping and lots of Smithsonian museums.

Rock Creek Hiker/Biker Trail - This trail totals 18.6 paved miles and is comprised of several linked paths that pass through Rock Creek Park. Along the trail are the National Zoo and the Equestrian Center at the Candy Cane City playground. This is one of the most popular trails in the DC metro area and has some grades, along with great scenic views. The so-called "urban" Rock Creek Park in DC encompasses more than 2,000 acres – more than twice the size of Central Park in New York City – extending 12 miles from the Potomac River to the border of Maryland. Established in 1890 by Congressional decree, the park includes more than 1,700 acres along the Rock Creek Valley, as well as having jurisdiction for several other city parks including the Glover Archbold Park, Montrose Park, Dumbarton Oaks Park, Meridian Hill Park, Battery Kemble Park, Palisades Park, and Whitehaven Park, among others.

Washington & Old Dominion Trail - The 45-mile Washington & Old Dominion Trail (W&OD, also affectionately called "Wad") is one of the supreme trails in the area, a "rails to trails" conversion with a smooth, paved surface and a painted yellow center line. The W&OD Trail starts in the Shirlington area of Arlington County, just off I-395 Exit 6, and ends in rural Purcellville, VA. Along its length are several towns including Falls Church and Leesburg. This is a great trail but can get quite crowded and it's also somewhat marred by the power lines. Note that a round-trip would require 90 miles on a bike, so if you have a couple days, consider some of the B&Bs along the way.